Hello K2PI,
That’s an amazingly clean trace, indeed; thanks for sharing it!
Since they are coincident in time (the sound and seismic waves), I think the most plausible explanation is that the same ground motion that set the geophone moving also set the BOOM sensor moving, which generated its own pressure waves!
This topic from some years ago covers very similar detections: Boom detects EQ 271 km away
Another very interesting case!